In a clip filmed outside the building, which houses the Canadian High Commission, Johnston can be seen touching the Queen’s arm as she walks down a set of steps.
"I’m certainly conscious of the protocol," David Johnston told CNN partner CBC News. "I was just anxious to be sure there was no stumbling on the steps."
" I thought perhaps it was appropriate to breach protocol just to be sure that there was no stumble," he explained.
Although the Royal Family’s official website says that "there are no obligatory codes of behavior" during a meeting with the Queen, touching the monarch beyond shaking hands is unusual and guaranteed to generate headlines.
In 2009, when then US First Lady Michelle Obama embraced Queen Elizabeth, the gesture caused astonishment in the British media, although Buckingham Palace later clarified that there had been no breach of protocol, calling the hug a "mutual and spontaneous display of affection and appreciation".